Can energy double?

[Zhang Yalin]

The device can't be changed, otherwise, 100=100.

[Bentoptics]

Unfoturnatly if the mass of the car equals the mass of the falling weight
There will be no motion at all. Friction will prevent any movement.
Look over the calculations in the attached file. Note I did not add the
Account of friction that will be present in the pulley guiding the string so
The losses will be higher. I would suggest you to build it and try it out
I think there is a lot to learn from trial and error.

BentOptics

[Zhang Yalin]

No, you're wrong in the last step.
The ball and the car have the EK(80J) at the end.

[FreeEnergy]

the weight going horizontally must have wheels... 

[Zhang Yalin]

Of course, it has wheels.